package LeetCode.LC105;

/**
 * @author corn
 * @create 2020-08-12-20:09
 */

import java.util.HashMap;

/**
 * 105. 从前序与中序遍历序列构造二叉树
 * 根据一棵树的前序遍历与中序遍历构造二叉树。
 * 注意:
 * 你可以假设树中没有重复的元素。
 * 前序遍历 preorder = [3,9,20,15,7]
 * 中序遍历 inorder = [9,3,15,20,7]
 */
public class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return helper(inorder, preorder, 0, 0, inorder.length - 1);
    }

    public TreeNode helper(int[] inorder, int[] preorder, int preStart, int inStart, int inEnd) {
        if (inStart > inEnd) {
            return null;
        }
        //找到当前根结点的值:即前序数组的首数值。
        int currentVal = preorder[preStart];
        //创建当前根结点
        TreeNode current_root = new TreeNode(currentVal);
        //在中序遍历中找到currentVal的下标
        int inIndex = 0;
        for (int i = inStart; i <= inEnd; i++) {
            if (inorder[i] == currentVal) {
                inIndex = i;
            }
        }
        //整理左右子树
        TreeNode left = helper(inorder, preorder, preStart + 1, inStart, inIndex - 1);
        TreeNode right = helper(inorder, preorder, preStart+(inIndex - inStart)+1, inIndex + 1, inEnd);
        //连接左右子树
        current_root.left = left;
        current_root.right = right;
        //返回当前结点
        return current_root;
    }
}
class TreeNode {
       int val;
       TreeNode left;
       TreeNode right;
       TreeNode(int x) { val = x; }
   }
